Fertility question...

About how long is it after a hen starts laying before the eggs are fertile? I have 3 hens and 2 roosters. My hens have been laying for about a month now. I have a friend at work who wants some chicks so I told her I'd try to hatch some for her....We only have about 5 weeks left in school so I need to get some started before too long if I'm gonna get them to her before we go home for the summer. I THINK we found one that was fertile when we ate some about a week ago....But it only looked like it with one.....I kinda thought if one is, they all should be? I've heard "if you have hens and roosters, the eggs are fertilized!" but don't know if that's exactly true.....Is putting some in the bator and candling after a few days the best way to find out?